I bench ran a Cox .15 Medallion last week and it had much vibration.This leads me to a basic question - is the prop shaft balanced, in general, in engines?And, then, has anyone balanced one of these shafts for the .15 Medallion?Having a way to test balance is the first objective.The second is then machining it in a lathe to achieve balance.Cheers,Peter

Peter, I have seen this many times and it was a very common problem with the Medallion .15. Make certain the piston isn't coming above the glow plug landing. I bet it is by a tiny bit. The solution is to add another head gasket or use a Fox .35 head gasket and shim the cylinder higher. You can place a wire laying it on the plug landing with the plug removed and rotate the engine and see if it pushes the wire off of the landing. However, excessive ball socket play if it is a ball socket version and not a conrod is unrecognized here and that play in itself can cause the piston to just slightly strike the plug causing the problem your experiencing. Ken

Ken has more experience with the various engines so listen to and re read what he said.

The basic question begs a bit more thought,,,,ALL single cylinder engines are going to have a lot-- or SOME imbalance ----just simply due to no real way to counter balance all the rotating parts ---for all RPMs ... again.... for ALL RPMs

Add in the dynamics of a propeller--- within the power stroke ----(given we have perfectly balanced the prop) ----and we actually can mathematically determine the rate of imbalances based on all the rotating mass

BUT who the hell, besides the engine testing guys, wants to do all that

I was taught, at an early age, to always clock the prop so that it was horizantal (against compression) when the engine quit

That way, landing (without LG and wheels ) would not have a prop blade digging in

And to this day I still clock all my combat prop engines for horizontal stop

Some may have noted I bench run ALL my engines...several reasons...NONE really about break in

ONE distinct reason is balance. Especially for Landing Gear planes.... in this case prop stop orientation is NOT critical but vibration of the total air-frame IS

I always start with the prop set to good horizontal stop against compression with engine quit

BUT---- IF on the bench, I have noticeable vibration, I do play with the prop clocking to reduce or eliminate the vibration

I have read--- but do not condone...more than one old guy who uses deliberately OUT of Balance Prop blades to Tame Fox .35 vibrations...NOT condone, because there is simply no way to repeat the condition

Thanks for fast replies with some new info to me.I did balance the prop so that is not part of the equation. But I can recheck.The piston/head bump issue I can check when I get back to the engine next week.I do not believe there is any piston-wristpin-connecting rod slop that I can detect by a wiggle test. I will have to check whether it is a wrist pin or socket version.I've heard of the Fox offset-imbalanced-prop trick, and should have thought of doing a little partial rotation test with the prop now no it - that good tip is noted for the future and much appreciated.

PS - I just saw the Ringmaster-a-thon 2017 is planned for Sat & Sun 7 & 8 Oct and plan on getting this engine on a RM Jr.I'll have at least 3-4 1/2A or 1/2APlus Baby RM's and RM Jr.'s for that - hope you are all flying, too.I was just getting into the swing of things last year, and could not start my Tee Dee .09 RM Jr. before it got too dark - grrr.Since then I have fashioned my LiPo-driven, full digital readout GCS Charger - Glow Charger Supreme Charger.Because life is too short not too have good glow.Here's a non-close up of the charger with a Mark's Models Wanderer I was flying this summer with a Tee Dee .051,which has a black throttle body because that is what is available on eBay these days.Edit - and I was able to post a closer up pic of the GCS...